Field of the Invention
The present invention relates to computer memory, and more particularly to solid state equivalents of magnetic media hard disks.
Background of the Invention
Magnetic media hard disks enjoy widespread use in computer systems, and especially in personal computer systems and computer workstations. A typical current generation hard disk includes one or more rigid platters of a plastic material coated on both sides with metal particles. Typically, one magnetic head is associated with each platter surface, although more heads may be used to increase access speed at the cost of increased complexity and expense. The platters are caused to spin at a high speed, typically 3600 rpm or greater, and the heads are designed to float just over the disk surfaces on a thin cushion of air caused by the spinning disks. The heads are moved across the spinning disk surfaces in a generally radially direction, for reading and writing magnetically encoded information from and to the disks. The heads are spaced just far enough away from the platter surfaces to avoid touching and damaging them, but are not so far away as to reduce signal strength and density. Since particulate matter in the air can enter the space between the heads and platter surfaces and cause damage to the platter surfaces, disk drives are assembled and sealed in a clean room environment, and the air within the disk chamber is microfiltered.
The physical organization of data on a disk is generally described by the terms track, sector, and interleave factor. "Tracks" are concentric rings of magnetically encoded data, and the track nearest the rim is generally designated track 0. A cylinder is a set of tracks having the same radial distance from the spindle. For example, in a two platter, two sided hard disk system in which the top side of the first platter is side 0, the bottom side of the first platter is side 1, the top side of the second platter is side 2, and the bottom side of the second platter is side 3; the four tracks 0 associated with sides 0, 1, 2 and 3 are collectively known as cylinder 0. The number of bytes in a track depends on the data encoding scheme used, examples of which include "modified frequency modulation," or "MFM," and "run-length limited coding," or "RLL." A "sector" is a segment of a track that is read or written at one time. The number of bytes in a sector depends on the operating system for which the manufacturer is designing; for example, DOS uses 512 bytes per sector. Contemporary disks are generally soft sectored, meaning that a sector address is written into the sector itself during disk physical formatting and the sector is identified by its address. Other information written during physical formatting includes synchronization bytes, which generally follow the sector address, and filler bytes, which are placed between sectors for timing tolerance. An "interleave factor" is the number of sectors inserted or interleaved between sequentially addressed sectors. An interleave factor of 1:1 means that sectors with sequential sector addresses are written in sequential track sectors. An interleave factor of 2:1 means that sectors with sequential sector addresses are written in every second track sector.
Operating systems such as DOS, which is used on personal computers from International Business Machines, Inc. and compatible personal computers, use logical sector numbering, which is a sequential or one-dimensional sector numbering scheme. While the DOS sector numbering scheme differs from the sector identification scheme used by the disk controller, this problem has been overcome by the use of built in ROM-BIOS that translates between the different schemes and issues calls that specify disk locations by side, track, and sector.
A disk must be physically formatted, and then logically formatted, before it can be used. Physical formatting, also known as low level formatting, involves formatting each track in accordance with the appropriate sector size and interleave factor. Physical formatting is to be distinguished from logical formatting, which is the organization of data on the hard disk and-the records, tables, and directories established to find the data.
While disk capacity and performance is being continually improved, important hard disk factors such as access time, transfer rate, durability, power, and space requirements are closely related to mechanical and electro-mechanical aspects of the hard disk design, and economical improvements in these factors are difficult. For example, the access time of current generation magnetic media hard disks is typically no better than about 15 milliseconds.